1. Field of the Invention
The present invention relates to a microwave oven, and more particularly, to an apparatus for eliminating inrush current in an application of initial voltage in a microwave oven.
2. Description of the Prior Art
As well known, a microwave oven is a device for radiating microwave generated from a magnetron to a foodstuff safely placed in a cooking chamber and cooking the foodstuff by fictional heat generated by an inner molecular movement. This microwave oven is generally classified into two types; an electronic type using a microcomputer and a mechanical type using a timer.
FIG. 1 is a disassembled perspective view of a conventional electronic type microwave oven. Referring to FIG. 1, in the aspect of its structure, the conventional microwave oven 10 is roughly composed of a controlling part 20, a high voltage driving part 30 and a heating part 40.
The controlling part 20 has a plurality of buttons 21 for determining the cooking time and the cooking power in the microwave oven, and the plurality of buttons 21 are connected to a printed circuit board (PCB) 6 on the rear side of which a microcomputer 9 and various components are built-in. The microcomputer 9 of the PCB 6 performs the cooking mode by turning on/off the driving status of the microwave oven according to the time and power determined by the respective buttons 21. Further, the controlling part 20 includes a display window 24 for displaying various kinds of cooking times and cooking statuses, and a door lever 25, which will be described later, for opening and closing a door 43.
The high voltage driving part 30 consists of a magnetron MGT for generating microwave, a waveguide (not shown) for guiding the microwave generated from the magnetron MGT to a cooking chamber 41, a high voltage transformer HVT for generating high voltage and pre-heating voltage needed for the driving of the magnetron MGT, a cooling fan 3 for cooling the high voltage transformer HVT, a high voltage capacitor HVC, a high voltage diode HVD, and the magnetron MGT, and a lamp 2 for lighting the cooking chamber 41, etc.
The heating part 40 consists of the cooking chamber 41 as a closed space, a rotary tray 42 on which foodstuffs may be securely placed in the cooking chamber 41, and a door 43 for opening and closing the cooking chamber 41.
FIG. 2 shows the circuit diagram of FIG. 1. As shown in FIG. 2, first and second power lines L1, L2 for receiving general alternating current, AC 220V, are connected with both ends of a primary coil 7a of the high voltage transformer HVT via a noise filter 1. This noise filter 1 is composed of a main fuse FUSE 1, capacitors C1 to C3, an inductor L, a resistor R1, etc. A detailed description about operations of the noise filter is herein omitted since it is the same as a general noise filter for cutting off high frequency (HF) transferred to the outside through the power lines L1 and L2.
The first power line L1 is connected in series with a magnetron temperature sensor TCO1 which is turned on/off according to the temperature of the magnetron MGT to prevent overheating of the magnetron, a first door switch SW1 which is turned on/off according to the opening or closing status of the door 43, and a monitor switch SW.sub.MT for monitoring the opening or the closing state of the door 43.
The second power line L2 is connected in series with a cooking chamber temperature sensor TCO2 which is turned on/off according to the temperature of the cooking chamber 41 to prevent the overheating of the cooking chamber 41, and a power relay RY3 for turning on/off main power. The power relay RY3 is also connected in parallel with an inrush relay RY2 for eliminating inrush current. The inrush switch SW.sub.IR is connected to a resistor R2 for restricting the current, wherein the resistor R2 for current restriction is generally a cement resistor having a high resistance capability.
The power relay RY3 and the inrush relay RY2 constitute respective relay elements, together with individual coils, respectively. The power relay RY3 and the inrush relay RY2 are turned on/off by the respective relay coil which is disposed on the PCB 6 and controlled by the microcomputer 9.
At this time, the inrush relay RY2 is firstly turned on by a control of the microcomputer 9, and after a given time period, e.g., 20-40 ms, the power relay RY3 is operated. Therefore, after an application of initial power, the inrush current of high reverse-electromotive force generated by the high voltage transformer HVT is consumed by the current restriction resistor R2 and the inrush relay RY2, and then, the power relay RY3 is turned on, to thereby prevent an overload of each driving circuit and damage to the microwave oven.
For the operation of the microwave, the microcomputer 9 turns on the main relay RY4, the inrush relay RY2 and the power relay RY3 through the respective coils arranged on the PCB 6. Accordingly, the main relay RY4 is turned on, and the lamp 2, the fan motor 3 and the driving motor 4 of the microwave oven are operated. Further, as the inrush relay RY2 and the power relay RY3 are sequentially turned on, voltage is generated by the high voltage driving part 30, and the magnetron MGT of the high voltage driving part 30 generates the microwave for cooking foodstuff placed in the cooking chamber 41.
Meanwhile, the heater is operated by the same operation of the above components, except that the microcomputer 9 turns off the inrush relay RY2 and the power relay RY3, while turning on the heater relay RY1, to operate the heater 8 instead of the magnetron MGT so that the grill cooking is performed.
FIG. 3 is a disassembled perspective view of a conventional mechanical type microwave oven. In FIG. 3, in its structural aspect, a conventional microwave oven 10 may be roughly divided into a controlling part 20', a high voltage driving part 30, and a heating part 40, which is the same as in the above-mentioned electronic type. The difference of the mechanical type microwave oven from the electronics type is that the controlling part 20' is composed of respective time control knob 21 and power control knob 22 for determining each of the cooking time and cooking power of the microwave oven. Each of knobs 21 and 22 is connected to a timer 23 placed on its rear side. Due to the timer 23 which turns on or off the driving status of the microwave oven according to the time and the power determined by each of the knobs 21 and 22, the microwave oven performs a cooking operation properly. Further, the controlling part 20' also includes a display window 24 for displaying various cooking times and statuses, and a door lever 25 for opening or closing the door 43, which will be described later.
FIG. 4 is the circuit diagram of FIG. 3. As shown in FIG. 4, the first and second power lines L1 and L2 for receiving the general alternating current, AC 220V, are connected with both ends of the primary coil 7a of the high voltage transformer HVT via the noise filter 1. The noise filter 1 is composed of the main fuse FUSE 1, the capacitors C1 to C3, the inductor L, the resistor R1, etc. The detailed description about operations of the noise filter will be omitted since it is the same as a general noise filter used for cutting off the high frequency (HF) transferred to the outside through the power lines L1 and L2.
The first power line L1 is connected in series with the magnetron temperature sensor TCO1 which is turned on/off according to the temperature of the magnetron MGT to prevent overheating of the magnetron, the first door switch SW1 which is turned on/off according to the opening or closing status of the door, a power controlling switch SW.sub.VP for controlling the power of the microwave, and the monitor switch SW.sub.MT for monitoring the opening or the closing state of the door 43.
The second power line L2 is connected in series with the cooking chamber temperature sensor TCO2 which is turned on/off according to the temperature of the cooking chamber 41 to prevent the overheating of the cooking chamber 41, the second door switch SW2 which is turned on/off according to the opening or closing status of the door, a time switch SW.sub.TM for controlling the cooking time of cooking, and the inrush switch SW.sub.IR for removing the inrush current.
The other side of the monitor switch SW.sub.MT, one side of which is connected to the primary coil 7a of the high voltage transformer HVT, is connected to the second door switch SW2 through the monitor fuse FUSE2. The monitor switch SW.sub.MT is also connected in parallel with the inrush coil 6a for turning on/off the inrush switch SW.sub.IR. The inrush switch SW.sub.IR and the inrush coil 6a are constructed in one relay 6, and it does not operate immediately upon application of outside voltage, but operates after a lapse of a time period, e.g., 20-40 ms, based on its operating characteristic. The inrush switch SW.sub.IR is also connected in parallel with the resistor R2 for a current restriction, wherein the resistor R2 for current restriction is generally a cement resistor having high resistance capability.
Thus, when power is applied to the inrush coil 6b in an on-state of the monitor switch SW.sub.MT, the relay 6 turns on the inrush switch SW.sub.IR after a lapse of the time period based on its operational characteristic described above. Accordingly, immediately after an application of the initial power, the inrush current of a high reverse-electromotive force generated by the high voltage transformer HVT is consumed by the current restriction resistor R2, and then, the inrush switch SW.sub.IR is turned on, to thereby prevent overloading each driving circuit and any damage to the microwave oven.
The lamp 2, the fan motor 3, the driving motor 4 and a timer motor 5 are connected between the first power line L1 and the second power line L2. The lamp 2, the fan motor 3 and the timer motor 5 are driven generally by the power source voltage, AC 220V, and the driving motor 4 is driven by the low voltage, about AC 21V, supplied from one coil of the fan motor 3.
The time switch SW.sub.TM, the power controlling switch SW.sub.VP and timer motor 5 are housed in one body inside the timer 23.
As mentioned above, conventional microwave ovens of the electronic and mechanical types are very useful instruments, which accurately drive the high voltage driving part 30 in the time a user wants for respective foodstuff to be cooked and eliminates the inrush current upon an application of the initial voltage.
However, as mentioned above, the conventional electronic type microwave oven requires a plurality of expensive relays to eliminate any inrush current, and also requires some complicated microcomputer control technique to connect the individual relays in due sequence. Further, the overall manufacturing cost for the electronic type microwave oven and the mechanical type microwave oven increases due to an expensive relay which is required for an effective operation of the timer, and due to the very complicated relay installation and wiring processes during the manufacturing process.